Traditionally, thermodynamic power generation cycles, such as the Brayton cycle, employ an ideal gas, such as atmospheric air. Such cycles are typically open in the sense that after the air flows through the components of the cycle, it is exhausted back to atmosphere at a relatively high temperature so that a considerable amount heat generated by the combustion of fuel is lost from the cycle. A common approach to capturing and utilizing waste heat in a Brayton cycle is to use a recuperator to extract heat from the turbine exhaust gas and transfer it, via a heat exchanger, to the air discharging from the compressor. Since such heat transfer raises the temperature of the air entering the combustor, less fuel is required to achieve the desired turbine inlet temperature. The result is improved thermal efficiencies for the overall thermodynamic cycle. However, even in such recuperated cycles, the thermal efficiency is limited by the fact that the turbine exhaust gas temperature can never be cooled below that of the compressor discharge air, since heat can only flow from a high temperature source to a low temperature sink. More recently, interest has arisen concerning the use of supercritical fluids, such as supercritical carbon dioxide (SCO2), in closed thermodynamic power generation cycles. One such prior art system 1 is illustrated in FIG. 1.
As shown in FIG. 1, the prior art power generation system 1 includes compressors, turbines, combustors and heat exchangers arranged in a first Brayton cycle 402, in which the working fluid is a supercritical fluid, and a second Brayton cycle 404, in which the working fluid is ambient air. The system 1 therefore includes an SCO2 cycle flow path 406 and air breathing cycle flow path 423, which may be separate from each other.
In FIG. 1, the flow of SCO2 along flow path 406 is as follows. Initially, a stream A of supercritical fluid is supplied to the inlet of a compressor 408. The supercritical fluid enters the inlet of the compressor 408 after it has been cooled and expanded to a temperature and pressure that is close to its critical point. The supercritical fluid is supplemented by a supercritical fluid source 431. After compression in the compressor 408, the stream B of SCO2 is heated in a cross cycle heat exchanger 410, which is connected to the SCO2 flow path 406 and air breathing flow path 423. The stream C of heated SCO2 from the heat exchanger 410 is then directed to the inlet of a turbine 412, where the SCO2 is expanded and produces shaft power that drives both the SCO2 compressor 408 and an output device 416 by shaft 417. The output device 416 can be a turboprop, turbofan, gearbox or generator. After expansion in the turbine 412, the stream D of SCO2 is cooled in a second cross cycle heat exchanger 418, also connected to the SCO2 flow path 406 and air breathing flow path 423. The stream A of cooled SCO2 is returned to the inlet of the compressor 408 via the flow path 406. In the air breathing Brayton cycle 404, initially, ambient air 411 is supplied to a compressor 420. The stream E of compressed air from the compressor 420 is then heated in the heat exchanger 418 by the transfer of heat from the SCO2 after the SCO2 has been expanded in the turbine 412. The stream F of heated compressed air is then directed to a combustor 424. The combustor 424 receives a stream 427 of fuel, such as jet fuel, diesel fuel, natural gas, or bio-fuel, is introduced by a fuel controller 428 and combusted in the air so as to produce hot combustion gas. The stream G of the combustion gas from the combustor 424 is directed to the heat exchanger 410 where heat is transferred to the SCO2, as discussed above. After exiting the heat exchanger 410, the stream H of combustion gas is expanded in a turbine 426, which produces power to drive the air compressor 420, via shaft 421. After expansion in the turbine 426, the combustion gas I is exhausted to atmosphere.
While the supercritical-ambient fluid cycle power generation system 1 shown in FIG. 1 can be advantageous, the heat exchangers required to transfer heat between the supercritical fluid cycle and the ambient cycle may be large, expensive, and impractical to implement. More effectively managing flow cycles can improve heat transfer efficiency in power generation systems that employ supercritical fluid cycles.